the Alps' largest and longest glacier, lying in the Bernese Alps of south-central Switzerland. Covering an area of 66 square miles (171 square km), it is divided into the Great Aletsch (main) and the Middle and Upper Aletsch (branches). The main glacier is 15 miles (24 km) long and 1 mile (1.6 km) wide. It extends generally southward from the Concordia Platz (where several other glaciers meet) to the Aletsch Forest (a nature reserve). Descending from the Aletschhorn (peak; 13,763 feet 【4,195 m】), the Middle Aletsch reaches the main glacier nearly opposite Märjelen Lake, a small lake bordering the Great Aletsch and lying just north of the Eggishorn. The Massa River, a tributary of the Rhône River, issues from the Great Aletsch. Skiing, mountain climbing, and glacial excursions are popular in the region.